1. Field of the invention
This invention relates to a permanent magnet motor including a rotor composed of permanent magnets and a washing machine provided with the permanent magnet motor, and more particularly to such a permanent magnet motor including a stator core improved for the purpose of reducing a cogging torque and such a washing machine in which vibration and noise due to operation thereof is reduced.
2. Description of the prior art
Various configurations of stator cores have conventionally been provided for improving starting characteristics of permanent magnet motors. For example, Japanese Unexamined Utility Model Reg. Publication No. 2-22096 (1990) discloses a brushless motor provided with a stator core having such an improved configuration. FIG. 8 shows the stator core disclosed in the publication. The shown configuration of the stator core reduces a cogging torque as well as simplifies a starting circuit arrangement.
Referring to FIG. 8, an electric motor of the inner rotor type in which a rotor 1 is disposed inside a stator is shown. The rotor 1 includes rotor magnets 2 comprising permanent magnets. The stator includes a stator core 3 having circumferentially alternately arranged first and second teeth 4 and 5. Each first tooth 4 includes a head 4a having an end face opposed to the rotor magnets 2. Each tooth 5 also includes a head 5a having an end face opposed to the rotor magnets 2. The end faces of the heads 4a and 5a are formed into the shape of an arc about a center of rotation of the rotor 1 and have different radii which are distances between the end faces and the center of rotation of the rotor 1. Accordingly, an air gap gl between the head 4a of each tooth 4 and the rotor magnets 2 differs from an air gap g2 between the head 5a of each tooth 5 and the rotor magnets 2, that is, g1&lt;g2. Armature windings are wound on the first and second teeth 4 and 5.
In the permanent magnet motors, a magnetic energy density in the air gap is higher in the vicinity of the teeth of the stator core and lower in the vicinity of slot openings between adjacent teeth. Furthermore, the magnetic energy density is lower in an area of the air gap located at each boundary between the magnets of different magnetic poles abutting each other than in the other area of the air gap. As a result, when the boundary between the magnets of different magnetic poles passes the opening between the adjacent teeth, portions of low magnetic energy at the rotor side and the stator core side coincides with each other such that the cogging torque shows a peak value. Since the peak of the cogging torque simultaneously occurs at a plurality of openings between the teeth, influences of the cogging torque become conspicuous.
The air gaps g1 and g2 differ from each other in the construction as shown in FIG. 8. As a result, the cogging torque does not take the peak values at all the openings 6 simultaneously and accordingly, the peak value of the cogging torque is decreased. However, each first tooth 4 and each second tooth 5 differ in magnetic resistance from each other since the air gaps g1 and g2 differ from each other. Consequently, the differences in the magnetic resistance vary an electromagnetic force when the armature windings are energized so that the rotor 1 is rotated. This results in vibration and noise in the motor.